Patients suffering from trauma or disorders causing severe joint pain often require surgical treatment involving complete or partial replacement of the affected joint. For instance, disorders of the gleno-humeral or shoulder joint may require the surgeon to replace the components of the patient's joint with prosthetic parts. During a shoulder replacement operation, at least a portion of the proximal section of the humeral shaft is replaced by a metal prosthesis. This prosthesis generally consists of two parts: a stem that is mounted into the medullary canal of the humerus, and a head retaining component connected in some manner to the stem. The head retaining component replaces the bearing surface of the humerus and articulates within the glenoid cavity of the scapula to allow movement of the shoulder.
In such operations, surgeons increasingly want to be able to custom fit prostheses to patients without having to choose a properly sized prosthesis from a large group of preformed one-piece, or “monobloc,” implants. Use of monobloc implants requires the surgeon to maintain a large inventory of such implants and allows the surgeon to custom fit the implant only within pre-set parameters.
Modular prostheses are known where the stem and head retaining component may be supplied in “modular” form, that is, as separate connectable components. Different stem sizes and head sizes in modular implant design provide the surgeon with some degree of inter-operative flexibility. A range of stem and head sizes allows the surgeon to choose a particular combination to suit the anatomy of each individual patient without having to maintain a large inventory of integral or “monobloc” prostheses. However, while these prior modular prostheses accommodate variations in patients' bone structure that arise due to differences in bone size and shape, they do not address variations in the angles and orientation of the skeletal system among individual patients. For example, in the case of the proximal humerus, individual patients require differing resection angles, i.e., angles of inclination of the humeral head relative to the stem, differing angles of retroversion or anteversion of the head, and differing degrees of lateral and/or posterior offsets between the axis of the head and the axis of the stem.
Other modular humeral prostheses have accommodated such variability between patients by a variety of adjustable connectors for linking the head to the stem or by surgical kits containing a wide range of connectors from which to select that fix the appropriate offset, resection angle, and angle of anteversion/retroversion. However, these solutions have drawbacks in that either they require a large number of connectors for mixing and matching with the various sized stems and heads, or the adjustable connectors are limited to adjustment in specific increments or enable adjustment of only a single variable or along only one or two rotational axes.
Prior humeral prosthetic implants have used ball joints in place of the traditional taper lock, or “Morse taper,” design for connecting the humeral stem and humeral head to provide a continuous range of possible arrangements of the head with respect to the stem. This type of joint makes it possible to define the most favorable resection angle, angle of anteversion and lateral offset of the humeral head. However, prior prostheses with ball joints secure the ball with various means requiring the use of small screws.
One prior art ball joint uses a set screw to engage a recess on the surface of the ball to hold the ball in place within a spherical cavity. This method of securing the ball is counter-productive because although the ball contains multiple recesses for engaging the screw, this arrangement limits the possible orientations of the ball in relation to the head and stem, thereby undermining the primary advantage of the ball joint—the capacity for continuous adjustment between the stem and head of the prosthesis.
Another example employs a slotted spherical ball and a screw plus a conical push rod designed to expand the ball to lock it within a spherical cavity in either the stem or the head. Such locking mechanisms have disadvantages in that the small blind hole required for the screws used in such mechanisms can collect debris which can fester and cause infection. Additionally, the small screws are difficult to handle during surgery and create a risk of becoming lost in the soft tissue of the patient. Furthermore, set screws concentrate the locking force on a small surface area rather than distributing it uniformly over a greater area. This concentrated stress can create more debris, thereby increasing the risk of infection.
Many prior prostheses incorporating a spherical ball joint have other drawbacks as well, such as difficulty creating a secure lock to prevent the components from slipping under pressure from high loads. This is most likely because prior ball joints house the ball in a generally spherical socket on either the humeral stem or humeral head. Such spherical sockets are designed to contour the geometry of the ball. This type of design has not demonstrated optimal torque to slip rates in laboratory studies designed to simulate the maximum expected torque loads on the joint during strenuous activities.
Thus, there is a need for a modular prostheses that allows continuous, three-dimensional variation of the orientation of the component parts, that reduces the number of standard component parts required to achieve optimal fixation, and that can be securely locked in a desired position without the use of small screws and with the ability to resist maximum expected loads without slippage. It would also be advantageous to have a basic prosthesis design that may be modified by using basic component parts that may be mixed and matched and then further adjusted relative to each other to allow reconstruction of the original anatomy of the patient. It would also be advantageous to provide a humeral prosthesis design made of modular parts which that may be independently selected to approximate the natural anatomy of the patient's proximal humerus for a shoulder replacement.